What is an atmospheric river, this potentially dangerous natural weather phenomenon?

What is atmospheric tea?

The term “atmospheric river” has been used in meteorology for only ten years. We speak of an “atmospheric river” when we have a rapid circulation of warm and moist air from tropical latitudes to temperate regions in the form of a fairly narrow corridor (200 to 400 km on average). Tropical air, which contains a lot of moisture, will be driven by a strong high-altitude jet stream. Encountering colder air, the trough will deepen and lead to an active disturbed system that will dump sustained heavy precipitation over a fairly geographically limited continental area.

“Pineapple Express” or “Rum Express”

While atmospheric rivers can cover large geographic areas, some regions, especially the west coast of North America and Europe, are more disturbed due to the prevailing ocean currents in these regions. For the North American continent, meteorologists use the term “Pineapple Express” or “Pineapple Road” in reference to the rapid circulation of tropical air from the Hawaiian Islands.

In Western Europe, meteorologists are more likely to use the term “Rum Express” when warm, humid air rises from the Antilles toward the European continent.

What are the consequences of an atmospheric river?

Atmospheric rivers play an important role in transporting and distributing moisture from the tropics to temperate and polar latitudes. They are in some cases beneficial to nature by bringing regular rains. They can also cause more or less severe weather with heavy rain or snow, flooding and flooding, strong winds, and damage to property and people.

At the end of last December, a tropical air mass with high humidity affected Brittany for several days. Finistère and Morbihan were on alert for heavy rains, flooding and flash floods, suffering from an onslaught of moderate but persistent rain. In the south of Finistere and west of Morbihan, up to 130 millimeters fell in 3 days, which is equivalent to 3 weeks of rain. These rains caused flooding and flooding, such as in Quimperlé, where the Leita River flooded part of the city.

Atmospheric tea Rum Express © Weather Channel

Some atmospheric rivers are more active, as in November 2021, when the Pineapple Express caused severe flooding in British Columbia, particularly in the Vancouver area. About 300 mm of rain fell in Umid settlement in 3 days, which is equivalent to a month and a half of rain. The city also broke the daily rainfall record with 174 mm, the old record being 133 mm on December 2, 1975. These torrential rains were followed by numerous flash floods and landslides, killing 6 people and causing several billion dollars in damage.

In early January, California suffered from very heavy rains, heavy snow in the Sierra Nevada and strong winds on the coast. depression bomb. Flooding continues and a state of emergency has been declared in California.

Will atmospheric rivers become more frequent with global warming?

The warmer the air masses are, the more moisture they can hold. According to IPCC scientists, a 1°C warmer atmosphere could hold 7% more water vapor. Thus, air masses from tropical latitudes moving towards temperate latitudes are able to hold more and more moisture and precipitation water. According to the latest IPCC report for 2021, the intensity and duration of precipitation events associated with atmospheric rivers can be expected to increase on the western fronts of Europe and the Americas.

Therefore, France may experience increasingly extreme events, with more frequent droughts associated with increased subtropical high pressures and anticyclonic blockages, but also with periods of heavy rain when these atmospheric rivers enter from tropical latitudes.

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